The diversity of known strategies for parasitic lifestyles is truly astonishing. Many species of parasitic worms, for example, utilize only one host species, while others cycle between two or more (as many as four) different species of hosts. Some parasites are highly virulent, seriously debilitating or even killing their hosts, while others cause only minor damage. Some parasites (such as viruses) are very small relative to their hosts and have the capacity for explosive reproduction. Others are almost as large as their hosts, and have relatively slow generation times. Therefore, parasites are difficult to categorize. Here, I use parasite to refer to organisms that have an obligate association with, and a negative effect on, another organism (the host). Host strategies for dealing with parasites are equally complex. Vertebrates have highly specialized immune systems that can rapidly respond to infection and then store information that can be used to mount future responses to the same type of infection. Invertebrates lack the memory cells of true immune systems, but they do have complex self-nonself recognition systems for recognizing and killing foreign tissues. Plants also have highly specialized defenses against pathogens, and the genetic basis of these defenses is especially well known due to the work of plant pathologists on crop plants. The myriad of details involved in the interactions between hosts and their parasites is overwhelming, but there are some shared, general aspects of these interactions that are of particular interest to evolutionary ecologists. First, parasites may attack in a frequency-dependent way. In other words, the probability of infection for a particular host genotype is expected to be, at least in part, a function of the frequency of that host genotype. This expectation has implications for sexual selection and the evolutionary maintenance of cross-fertilization (Sakai, this volume; Savalli, this volume). Second, parasites may affect the population density of their hosts, and host density may feed back to affect the numerical dynamics of the parasite. Host density may also affect natural selection on the reproductive rates of parasites, which in turn is likely to affect host fitness and host dynamics.
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